Fixed Linux problem

On the Linux download page at vpython.org there is a rebuilt source
package for VPython 5.72 which fixes the crashes on Linux (tested on
the latest Ubuntu).

The problem turned out to be that in src/gtk2/display.cpp the function
Gdk::GL::get_proc_address is broken on recent releases of Ubuntu and
Debian (and maybe Fedora) and has been replaced with glXGetProcAddress
which does work.

Bruce Sherwood

P.S. I'll mention that in my many years of work with VPython on three
different platforms, the Windows environment has been remarkably
stable across operating system changes, the Mac environment much less
so (in that until recently things broke when the OS version number
changed in the last decimal place), and Linux also has been somewhat
unstable.

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